Drill bits used to drill wellbores through earth formations generally are made within one of two broad categories of bit structures. Depending on the application/formation to be drilled, the appropriate type of drill bit may be selected based on the cutting action type for the bit and its appropriateness for use in the particular formation. Drill bits in the first category are generally known as “roller cone” bits, which include a bit body having one or more roller cones rotatably mounted to the bit body. The bit body may be formed from steel or another high strength material. The roller cones may also be formed from steel or other high strength material and include a plurality of cutting elements disposed at selected positions about the cones. The cutting elements may be formed from the same base material as is the cone. These bits may be referred to as “milled tooth” bits. Other roller cone bits include “insert” cutting elements that are press (interference) fit into holes formed and/or machined into the roller cones. The inserts may be formed from, for example, tungsten carbide, natural or synthetic diamond, boron nitride, or any one or combination of hard or superhard materials.
Drill bits of the second category may be referred to as “fixed cutter” or “drag” bits. Drag bits, include bits that have cutting elements attached to the bit body, which may be a steel bit body or a matrix bit body formed from a matrix material such as tungsten carbide surrounded by a binder material. Drag bits may generally be defined as bits that have no moving parts. However, there are different types and methods of forming drag bits that are known in the art. For example, drag bits having abrasive material, such as diamond, impregnated into the surface of the material which forms the bit body are commonly referred to as “impreg” bits. Drag bits having cutting elements made of an ultra hard cutting surface layer or “table” (such as made of polycrystalline diamond material or polycrystalline boron nitride material) deposited onto or otherwise bonded to a substrate are known in the art as polycrystalline diamond compact (“PDC”) bits.
PDC bits drill soft formations easily, but they are frequently used to drill moderately hard or abrasive formations. They cut rock formations with a shearing action using small cutters that do not penetrate deeply into the formation. Because the penetration depth is shallow, high rates of penetration are achieved through relatively high bit rotational velocities.
PDC cutters have been used in industrial applications including rock drilling and metal machining for many years. In PDC bits, PDC cutters are received within cutter pockets, which are formed within blades extending from a bit body, and may be bonded to the blades by brazing to the inner surfaces of the cutter pockets. The PDC cutters are positioned along the leading edges of the bit body blades so that as the bit body is rotated, the PDC cutters engage and drill the earth formation. In use, high forces may be exerted on the PDC cutters, particularly in the forward-to-rear direction. Additionally, the bit and the PDC cutters may be subjected to substantial abrasive forces. In some instances, impact, vibration, and erosive forces have caused drill bit failure due to loss of one or more cutters, or due to breakage of the blades.
In a typical PDC cutter, a compact of polycrystalline diamond (“PCD”) (or other superhard material, such as polycrystalline cubic boron nitride) is bonded to a substrate material, which may be a sintered metal-carbide to form a cutting structure. PCD comprises a polycrystalline mass of diamond grains or crystals that are bonded together to form an integral, tough, high-strength mass or lattice. The resulting PCD structure produces enhanced properties of wear resistance and hardness, making PCD materials extremely useful in aggressive wear and cutting applications where high levels of wear resistance and hardness are desired.
An example of a PDC bit having a plurality of cutters with ultra hard working surfaces is shown in FIG. 1. The drill bit 100 includes a bit body 110 having a threaded pin end 111 and a cutter end 115. The cutter end 111 includes a plurality of ribs or blades 120 arranged about the rotational axis L of the drill bit and extending radially outward from the bit body 110. Cutting elements, or cutters, 150 are embedded in the blades 120 at predetermined angular orientations and radial locations relative to a working surface and with a desired back rake angle against a formation to be drilled.
A plurality of orifices 116 is positioned on the bit body 110 in the areas between the blades 120, which may be referred to as “gaps” or “fluid courses.” The orifices 116 are commonly adapted to accept nozzles. The orifices 116 allow drilling fluid to be discharged through the bit in selected directions and at selected rates of flow between the cutting blades 120 for lubricating and cooling the drill bit 100, the blades 120 and the cutters 150. The drilling fluid also cleans and removes the cuttings as the drill bit 100 rotates and penetrates the geological formation. The fluid courses are positioned to provide additional flow channels for drilling fluid and to provide a passage for formation cuttings to travel past the drill bit 10 toward the surface of a wellbore (not shown).
A factor in determining the longevity of PDC cutters is the exposure of the cutter to heat. Exposure to heat can cause thermal damage to the diamond table and eventually result in the formation of cracks (due to differences in thermal expansion coefficients) which can lead to spalling of the polycrystalline diamond layer, delamination between the polycrystalline diamond and substrate, and conversion of the diamond back into graphite causing rapid abrasive wear. The thermal operating range of conventional PDC cutters is about 700-750° C. or less.
As mentioned, conventional polycrystalline diamond is stable at temperatures of up to 700-750° C. in air, above which observed increases in temperature may result in permanent damage to and structural failure of polycrystalline diamond. This deterioration in polycrystalline diamond is due to the large difference in the coefficient of thermal expansion of the binder material, cobalt, as compared to diamond. Upon heating of polycrystalline diamond, the cobalt and the diamond lattice will expand at different rates, which may cause cracks to form in the diamond lattice structure and result in deterioration of the polycrystalline diamond. Damage may also be due to graphite formation at diamond-diamond necks leading to loss of microstructural integrity and strength loss, at extremely high temperatures.
In convention drag bits, PDC cutters are fixed onto the surface of the bit such that a common cutting surface contacts the formation during drilling. Over time and/or when drilling certain hard but not necessarily highly abrasive rock formations, the edge of the working surface on a cutting element that constantly contacts the formation begins to wear down, forming a local wear flat, or an area worn disproportionately to the remainder of the cutting element. Local wear flats may result in longer drilling times due to a reduced ability of the drill bit to effectively penetrate the work material and a loss of rate of penetration caused by dulling of edge of the cutting element. That is, the worn PDC cutter acts as a friction bearing surface that generates heat, which accelerates the wear of the PDC cutter and slows the penetration rate of the drill. Such flat surfaces effectively stop or severely reduce the rate of formation cutting because the conventional PDC cutters are not able to adequately engage and efficiently remove the formation material from the area of contact. Additionally, the cutters may be under constant thermal and mechanical load. As a result, heat builds up along the cutting surface, and results in cutting element fracture. When a cutting element breaks, the drilling operation may sustain a loss of rate of penetration, and additional damage to other cutting elements, should the broken cutting element contact a second cutting element.
Additionally, the generation of heat at the cutter contact point, specifically at the exposed part of the PDC layer caused by friction between the PCD and the work material, causes thermal damage to the PCD in the form of cracks which lead to spalling of the polycrystalline diamond layer, delamination between the polycrystalline diamond and substrate, and back conversion of the diamond to graphite causing rapid abrasive wear. The thermal operating range of conventional PDC cutters is about 750° C. or less.